1. Field of the Invention
The present invention relates generally to a panel used for various types of surface light source devices such as a back light for a liquid crystal display, a display board, and a lighting display, and more particularly, to a panel for emitting light from the whole of a light emitting surface upon incidence of light emitted from a light source on a transparent substrate from an end surface of the transparent substrate and reflection of the incident light from a reflecting surface.
2. Description of the Prior Art
In various types of surface light source devices such as a back light for a liquid crystal display, a display board and a lighting display, a panel constituted by a transparent substrate having high light transmission properties is used in order that light is emitted from the entire surface of the surface light source device.
In thus emitting light from the surface light source device using the panel, a light source 1 such as a cold-cathode tube is provided in close proximity to an end surface 11a of the panel constituted by a transparent substrate 10 having high light transmission properties such as an acrylic board, and a light reflector 1a is provided around the light source 1, as shown in FIG. 1, so that light emitted from the light source 1 is introduced into the transparent substrate 10 from the end surface 11a. Although a case where the light source 1 is provided in close proximity to the end surface 11a of the transparent substrate 10 is only illustrated in FIG. 1, light sources may, in some cases, be respectively provided in close proximity to opposite end surfaces on both sides of the transparent substrate 10.
In emitting from a light emitting surface 12 the light introduced into the transparent substrate 10, the remaining end surface 11b, which is not in close proximity to the light source 1, of the transparent substrate 10 and a reflecting surface 13 on the opposite side of the light emitting surface 12 are respectively provided with reflection sheets 2, and the light introduced into the transparent substrate 10 is introduced into the light emitting surface 12 upon being reflected by the reflection sheets 2 so that the light is emitted from the whole of the light emitting surface 12.
In recent years, the following panels have been developed in order that light having uniform luminance is emitted from the whole of the light emitting surface 12 of the panel: a panel so constructed that irregular reflecting portions 13a in a circular shape for irregularly reflecting light are so provided on the reflecting surface 13 that the farther the irregular reflecting portion 13a is away from a light source 1, the larger the area thereof is, and a plurality of rows of irregular reflecting portions 13a are formed, as shown in FIG. 2, a panel so constructed that an irregular reflecting portion 13a is formed on the whole of the reflecting surface 13, as shown in FIG. 3, and a panel so constructed that a translucent diffusion sheet 3 is formed on the light emitting surface 12 for emitting light, and the light emitted from the light emitting surface 12 is made uniform upon being dispersed by the dispersion sheet 3, as shown in FIG. 1.
When the light emitted from the light source 1 is introduced into the transparent substrate 10 from the end surface 11a which is in close proximity to the light source 1, and the light is emitted from the light emitting surface 12 of the transparent substrate 10 in the above-mentioned manner, several bright lines called luminescent lines appear in the vicinity of the end surface 11a of the transparent substrate 10, generally in a position spaced approximately 5 to 10 millimeters apart from the end surface 11a and a position further spaced several millimeters apart from the position, and the luminance in portions on both sides in the direction of the end side of the end surface 11a is lower than that in the other portion.
When the panel is used for a surface light source device such as a back light of a liquid crystal display, therefore, there are some problems. For example, the display of images such as characters becomes unclear due to the luminescent lines. Further, the portions on both sides in the vicinity of the end surface 11a, which is in close proximity to the light source 1, of the transparent substrate 10 become dark, whereby the images such as characters become indistinct to read.
Furthermore, when irregular reflecting portions 13a in a circular shape are provided on the reflecting surface 13, as shown in FIG. 2, the reflecting states of the light in the irregular reflecting portions 13a and the other portion significantly differ. Therefore, the luminance of the light emitted from the light emitting surface 12 differs between the irregular reflecting portions 13a and the other portion, whereby a difference in brightness arises, and the irregular reflecting portions 13a are seen as if the pattern thereof was raised. On the other hand, when an irregular reflecting portion 13a is provided on the whole of the reflecting surface 13, as shown in FIG. 3, the irregular reflection of the light in the irregular reflecting portion 13a varies, whereby a subtle difference in brightness partially arises. Particularly when the thickness of the transparent substrate 10 used for the panel is small, the irregular reflecting portions 13a are significantly seen as if the pattern thereof was raised, or a difference in brightness significantly arises even when the translucent diffusion sheet 3 is provided on the light emitting surface 12 to diffuse the emitted light, whereby the images such as characters become indistinct to read.
In providing the reflection sheets 2 on the remaining end surfaces 11b, which are not in close proximity to the light source 1, of the transparent substrate 10 and the reflecting surface 13 as described above, the reflection sheets 2 have been conventionally affixed to the surfaces. However, work for affixing the reflection sheets 2 is very laborious and takes a lot of time. Further, the reflection sheets 2 are not satisfactorily affixed to the surfaces. The reflection sheets 2 are stripped from the surfaces by themselves while the panel is being used, whereby there are some problems. For example, the light leaks out of portions where the reflection sheets 2 are stripped, whereby the light introduced into the whole of the light emitting surface 12 is decreased. Further, the light is not reflected from the portions where the reflection sheets 2 are stripped, whereby the luminance in the portions is significantly made lower than that in the other portion.